Selenium cells



Sept. 1959 L. F. PEROTTE 2,

' SELENIUM CELLS Filed June 17, 1958 Fig. I 2

REsINous TRANSPARENT COATING COUNTERELECTRODE CADMIUM CONTACT STRIP CADMIUM NITRIDE RUBIDIUM SELENIDE I CRYSTALLINE SELENIUM Fig.2 IRON BASE ELECTRODE l mwawcwq L SELENIUM APPLIED To BASE PLATE 1 L HOT PRESS AT I80C j EXPOSURE TO METAL VAPOR FROM HEATING OF RUBIDIUM CHROMATE UNDER VACUUM L I-IEATING FOR ONE HOUR AT 2l8C j LAYER ADDED BY SPUTTERING CADMIUM IN LOW PRESSURE ATMOSPHERE OF NITROGEN AND ARGON L APPLICATION OF CONTACT STRIPS INVENTOR. Fig. 3

Laurence F. PeroHe His ArTorney .greater open-circuit ,voltages.

United States Patent G ,9 .3 E IU CELL Laurence F. Berotte, Arlington,tMass assignor to gen- Leral Electric ;Co1 npany,.a corporation of Newflork Thepresent invention relatesto.unilaterally-conducting electric circuit .elementsland,.more partiqula ly to selenium photovoltaic and rectifier cellshaving improved electrical. characteristics.

Alternating current rectifiers which .utiliz e .the .unidirectional conducting characteristics .of selenium are commonlyproduced by. applying. at least one. thin layer of selenium. to a .carrier plate which .also i serves as an electrode, byiforming a barrier layer atop thefselenium, and by affixing .a counter-electrodeto the assembly. Photovoltaic cells maybeinanufa ctured in a like manner with the counterelectrode element beiuglof a..lig ht-tra nsmitting character. In both the rectifier. photovoltaic units, certain electricalcharacteristics a 're found to be variable with time, temperatures, andilelectrical circuit conditions, and a relatively lengthy forming or aging process is generallyjemployed to aidin'festablishing initially high blocking..resistances and stabilityoffcharacteristics. The forming, process consumesvaluableproduction time and involves added handling .and.'. control.operations which are costly. Evenlthen it is foundthaflfwith time, cells may tend to exhibitfgre'atly altered. electrical characteristics, cau ing appreciable errors to ,occur. in the electrical circuits in which the cells areeinpIoyed.

Accordingly, it is an 'objectQof thepres'ent invention to provide a novel selenium cell assembly and ir'iethodj of manufacture to improve the electrical characteristics of selenium cells and toreduce the length of the formation process needed to produce useful output characteristics.

A further object'of the invention is to provide improved selenium cells wherein. such characteristics asblocking resistance, open-circuit voltage, and photo currentsare favorably affected by the formation of improyed' PfN deposit of cadmium nitride is formed which. very. greatly improves the n-type characteristicsof the cell, resulting in higher'blocking resistances and, in photovoltaiclcells, Furthermore, .the ..cells, which maybe in operative condition .even .without the usual aging operation, require very little additional aging to optimize their characteristics.

Although these teachings are directedtoward the formation of a particularnew barrier; layer .by a..novel process, 'it should not befeltv that the. practice of this invention necessarily excludes I the formation of other barrier layers .as well. Indeed, I have'found that, the best embodiment of this invention presently known makes use of an additional barrierlayerof diiferencescomposition. The provision of the'novel barrier layer which r 2,903,631 Ice Patented Sept. 8, 1959 is the subject of this invention in and of its elf works a great improvement in, the, characteristics of the selenium cell, but. the. combination. of this. barrier layer with others may result in even greater improvement.

As an aid in the understanding ,of this .invcntion,.referw m y b h to th co pa yin .dr wins l,- ths in= i u i a plan v w. o .a e e i m. o qvq ta ass formed according to these ,teachings; V

u .1 4 cro s-S t ona ew take 9. lin :Am of'Figure'; 1; .and, i

;Figure.3 is a -flow chart. illustrating themorqsalient aspects of the improved rnethoduby which.;the cell of Figures- 1 and 2 is produced.

As fir t nr ba v me 9 o qt e ts it le material is coated. withauniforrn layer- 2 of selenium. I prefer. to apply the selenium, by, ,eyaporating a. few. mils of selenium onto the. base, plate. in, a. vacu um chamber, atthoughfthere are many other familiar-ways to accomp is t u ubs quen l th in m t e ls a a nq mmcu ha b a ;.hot s toensurea ,uniforrnityof thickness of the selenium. 111c last-msntiq e s p .w q isl now i th w also be'carried out, if desired, after a-barrier layer .,is produced on the cell, though l prefento perform it,,prev pu tqth omle io of an b r l y t sausap the possibility. of damage to these layers.

Before the. conversion of the selenium,. layer;.-2 i nto,a s r s alli stat t ha e. fqn tde r ble, a thoushm a ss u l ece s 10 qd a an i i lccki nl y 3 rubi i m .ss ta p .p tiqn llw of .um ise en d s r a e-d..- h .ssbniflmlay .Thwmd st c fif h vb ms lsnis v i ss snid b wk laye is not the subject of this application but is described in my. cope ndingapplication entitled-Selenium, Cells, Serial No 142,557, filed o f ey en. date herewith and, assigned to the same assignee ,as .that ,of..the present application. As is vmore ffully setl forth, in the aforesaid .application,, the r b um. t e n s layer. is rms na acnumibr vaporating. onto the e) osed selenium. surface a. thin layer of .r'libiiii'limlwfivrted into. tha a o stet bis-heat n rubid- 1 e qi e Em t qsss qse .Pssa s the est r o-positivenature ofrubidium, a reaction talces place on contact with Ithe .iscleniiim t0 93 guhidiu selenidc. h r ft l :mmr s smamlay i c ony erted to a crystalline istate-in a.. manner ,kno wn in the art .by', heating. the. incomplete .cell; at a. temperature of approximately .218 C. ;f or:about an-fhour.

' According to. the.presentYdisclosuraan additional layer w b't v 1 i d rumt nsei ths dde to the. se lenium, cell. ,"Imthe productionpf layer 4 selenium cell is .placed [in a vacuum chamber wlneh s evacuatedia'nd onto which. a vmi tture fo f {gases including nitrogen, e s sentia lly, andlargom preferably in a rat about one to. one atlow pressuresi ,The p referred pressures are about 115 microns fjiareach' gas. In thisatmosphere a few brief electricalldischarges aremade between a .cadmiurricathode .an the selenium cell ,as,,an ano a a re t wh dys t i 'l ysrfii ut re .onto the cell. Empirical evidenee sugge .ts that this ,layer is cadmium nitride, and for this reason as wellas for convenie nce, I prefer to designate. .cells treated in this .way as nitrided.cells, or. as having nit r i ded j unc tions. It,.i s essential that no. oxygen} or I water vapor ,be ..pre sent in 'the sputtering atrn'c spliere,- since .thelpr esenceof either 'inhibitsthe 'formationfof the nitride. f

It is found that di ringthe p'rocessnot only is,the,.cad minim electrode eroded, but nitrogen isalsoconsumed,

f a's evidenced by 1 the fact. that the. nitrogen pressure ;d

creases and the jargon pressure remains .the same. 1 ;In

of cadmium'electrodes, only platiritiinwas deposi d,. o n

aeoaeer stantially constant. This strongly suggests that during the sputtering process the nitrogen removed from the mixture of gases reacts only with the cadmium and not with the selenium.

Further evidence for the formation of a nitride of cadmium was obtained on decomposing a cadmium layer sputtered onto glass slides in a nitrogen-argon mixture by heating the slides in a high vacuum. The pressure rise was greater for those with nitrided layers than for glass blanks or for those with cadmium layers obtained by sputtering cadmium in an atmosphere of pure argon. This greater pressure rise can only be due to the nitrogen.

Although layer 4 may be deposited in an atmosphere of pure nitrogen, I have found that the process then takes considerably longer. The process of sputtering is essentially a momentum exchange phenomenon, the ions of the gas in which the discharge takes place acting on the cathode to knock loose metallic ions which travel to the anode. The addition to the nitrogen atmosphere of an inert gas, especially one of a reasonably high atomic weight, appears to result in the formation of a cadmiumrich ionic layer about the cathode which reacts readily with the nitrogen present. The nitrogen ions, being of a lower atomic weight, would not be able to knock loose cadmium ions as readily as would the heavier argon ions, hence the reason that the use of a pure nitrogen atmosphere requires longer periods of sputtering to produce an equivalent blocking layer. As a constituent of the nitrogen atmosphere, argon is selected because of the fact that, among the heavier inert gases, it is relatively inexpensive.

Cadmium contact strips 5 are then placed on the cell. To the cadmium nitride layer 4 and contact strips 5, a counterelectrode is then applied, and this may take the form of sputtered alternate layers of cadmium and platinum. In the case of a photovoltaic cell the counterelectrode should be sufliciently transparent to admit light to the PN junction, and it is, therefore, made quite thin. In a photovoltaic cell I prefer to employ a known type of counterelectrode incorporating cadmium oxide as a major constituent because it is both transparent and relatively conductive. In either event, good electrical contact should be made between the contact strips and the counterelectrode, and between the counterelectrode and the underlying surface of the cell. The oxide may be formed by sputtering the cadmium in an atmosphere containing suflicient oxygen to oxidize most, but not all, of the cadmium, since an excess of cadmium improves the conductivity of the oxide layer. Argon may also be present in the oxygen atmosphere to improve the momentum exchange as referred to above in connection with the description of the nitrogen atmosphere sputtering. Some metallic cadmium is generally intended to be deposited because of the additional reason that, even with the interposition of a blocking layer, subsequent aging or forming of the cell causes a migration of some atomic cadmium to the selenium layer where it reacts to form cadmium selenide which enhances the characteristics of the PN junction.

The last layer applied is a layer 7 of a clear resin which serves along with the counterelectrode to protect the cell against the absorption of gases or other substances which might adversely affect the cells electrical characteristics.

Most selenium photovoltaic cells, especially those having metallic cadmium in the counterelectrode, require a final aging operation at elevated temperatures. This aging operation has two effects, one. of which is to thin the counterelectrode so as to permit it to admit more light and consequently increase the current output of the cells. Simultaneously, the aging operation permits a reaction product to form between the cadmium containing counterelectrode and the selenium of the cell to produce a natural blocking layer. This aging operation generally consumes a great amount of time and it is manifestly desirable to reduce the time required to complete the development of the cell characteristics. Within limits, increasing the temperature at which the aging is carried out reduces the amount of time required, but when the temperature exceeds C. by very much, the cell can easily be ruined by the diffusion of the different materials into each other resulting in the destruction of the blocking layer and, hence, deterioration of the PN junction.

Once the counterelectrode has been applied to a cell formed as described herein, however, the cell is found to be in operative condition even without the customary aging operation and its characteristics are equal to or better than otherwise equivalent aged cells not having the blocking layer of cadmium nitride. This fact of itself establishes that a good PN junction has already been formed. Nevertheless, subsequent aging of the cell does result in optimizing its characteristics, suggesting that some cadmium has migrated through the previously formed blocking layer to form a small amount of cadmium selenide which further improves the nature of the PN junction. The nitrided junction, however, appears to prevent the deterioration of the PN junction by the migration of too much cadmium into the selenium layer. I have found that nitride cells of the nature described above, either with or without the addition of a rubidium selenide or cesium selenide layer, when aged at to C. for as little as 30 hours, exhibit optimum characteristics much better than those of otherwise equivalent cells which are not nitrided. Whereas at these high aging temperatures, most other cells would be destroyed, I have also found that the usual aging temperatures of 75 to 100 C. effect only a slight improvement in the characteristics of nitride cells.

The following table comparing the characteristics of cells described herein with those of otherwise equivalent standard cells employing as a blocking layer only the PN junction which results from the contact between the selenium layer and a cadmium-containing counterelectrode should serve to emphasize the improvements afforded by a practice of these teachings. The values given are averages of a representative number of cells and the potential and current measurements were made at 60 footcandles of illumination.

manufacture, instead of an aging operation, an electricalforming treatment. This treatment consists of subjecting the cell to an alternating or direct current until a high resistance is developed in the reverse direction. The forming treatment has an object similar to that of the aging treatment in that it results in gradually building up a blocking layer. The forming treatment has generally required several hours for its completion. By a practice of this invention, the basic PN junction is already avail able and such forming as may be required to develop optimum cell characteristics may be completed in a matter of minutes instead of hours. The forming process does, however, result in a slight increase in the potential which the nitrided rectifier cell is capable of withstanding.

From What has been set forth above, it will be apparent that the establishment according to these teachings of a nitride n-type blocking layer may be independent of the formation of other layers either between it and the selenium or between it and the counterelectrode. This invention may be practiced by forming the cadmium nitride either alone with no other n-type layers or in combination with others. In combination with other layers, however, there will probably be a particular order in which the layers should be formed to obtain an optimum P-N junction. Thus, I have found that better results are obtained by placing the cadmium nitride layer upon the rubidium selenide layer as indicated by the results summarized in the table instead of vice versa. This suggests that, even as thin as these layers may be, separate startiform layers are formed instead of a single layer of intermixed constituents.

It should thus be apparent that the examples given are illustrative in nature and not necessarily limiting of the scope of the invention in its broader aspects as determined by a fair interpretation of the appended claims.

What I claim as new and desire to secure by Letters Patent of the United States is:

1. A unilaterally conducting circuit element comprising a conducting electrode having a layer of crystalline selenium thereon, a counterelectric for making electrical contact with said selenium layer, and a blocking layer comprising cadmium nitride interposed between said counterelectrode and said selenium layer.

2. A unilaterally conducting circuit element comprising a conducting electrode having a layer of crystalline selenium thereon, a counterelectrode for making electrical contact with said selenium layer, and a blocking layer interposed between said selenium layer and said counterelectrode comprising a material formed by exposing said circuit element to cadmium sputtering in an atmosphere consisting essentially of nitrogen.

3. A unilaterally conducting circuit element comprising a conducting electrode having a layer of crystalline selenium thereon, a counterelectrode for making electrical contact with said selenium layer, and a blocking layer interposed between said selenium layer and said c0unter electrode comprising a material formed by exposing said circuit element to cadmium sputtering in an atmosphere consisting essentially of a mixture of nitrogen and an inert gas having a higher atomic Weight than that of nitrogen.

4. A unilaterally conducting circuit element comprising a conducting electrode having a layer of crystalline selenium thereon, a counterelectrode for making electrical contact with said selenium layer, and a blocking layer interposed between said selenium layer and said counterelectrode comprising a material formed by exposing said circuit element to cadmium sputtering in a dry oxygenfree atmosphere consisting essentially of a mixture of nitrogen and argon.

5. The process of manufacturing a unilaterally conducting circuit element having a conducting electrode, a selenium layer thereon, and a counterelectrode which includes the step carried out prior to the application of said counterelectrode of producing an n-type layer on said circuit element by sputtering it with cadmium in a low pressure atmosphere consisting essentially of nitrogen.

6. The process of manufacturing a unilaterally con ducting circuit element having a conducting electrode, a selenium layer thereon and a counterelectrode which includes after the formation of said selenium layer but before the formation of said counterelectrode the steps of placing said circuit element in an evacuated chamber containing a gas consisting essentially of nitrogen under low pressure, initiating an electrical discharge while in said gas between a cadmium cathode and said circuit element as an anode, whereby an n-type blocking layer is formed on said circuit element.

7. The process of manufacturing a unilaterally conducting circuit element having a conducting electrode, a crystalline selenium layer thereon and a counterelectrode which includes after the formation of said selenium layer but before the formation of said counterelectrode the steps of placing said circuit element in an evacuated chamber containing a gas consisting essentially of nitrogen and an inert gas having an atomic weight higher than that of nitrogen under low pressure, initiating an electrical discharge while in said gas between a cadmium cathode and said circuit element as an anode, whereby an n-type block' ing layer is formed on said circuit element.

8. The process of forming a blocking layer on a selenium cell before the application of a counterelectrode thereto comprising the step of exposing said cell to cadmium sputtering in a dry oxygen-free atmosphere consisting essentially of nitrogen.

9. The process of forming a blocking layer on a selenium cell before the application of a counterelectrode thereto comprising the step of exposing said cell to cadmium sputtering in a dry oxygen-free atmosphere consisting essentially of nitrogen and an inert gas having an atomic weight higher than that of nitrogen.

Ruben Mar. 18, 1930 Addink June 6, 1950 

1. A UNILATERALLY CONDUCTING CIRCUIT ELEMENT COMPRISING A CONDUCTING ELECTRODE HAVING A LAYER OF CRYSTALLINE SELENIUM THEREON, A COUNTERELECTRIC FOR MAKING ELECTRICAL CONTACT WITH SAID SELENIUM LAYER, AND A BLOCKING LAYER 